Known in the art are offset printing plates made of aluminium substrates. The non-printing areas of such substrates are made of oxidized aluminium and as the printing areas use is made of a material of a light-sensitive top layer based on derivatives of ortho-naphthoquinonediazide-sulphonic or carboxylic acids or a material of a light-sensitive top layer comprising a photopolymerizable composition. The printing areas of these plates are positioned above the non-printing areas, their height being 2-3 .mu.m(cf. FRG Pat. No. 2,622,799 Cl. G 03 F-H 02; French Pat. No. 2,157,645 Cl. B 41 n 1/00, B 41 n 3/00; FRG Pat. No. 1,238,048 Cl. 15 l 2/02, B 41 n; FRG Pat. No. 1,546,786 Cl. 15 l 2/02, B 41 n 1/08; "Operation Instructions of Offset Printing", "Kniga" Publishing House, Moscow, 1970).
The above-described prior art offset printing plates have an insufficient mechanical strength which is due to a low mechanical strength of an aluminium substrate (tensile strength is 14 to 20 kgf/cm.sup.2). This restricts the field of application of these offset printing plates on all kinds of printing equipment, especially web presses. Furthermore, non-printing areas made of oxidized aluminium have a relatively low wear-resistance (the volume of ground-off material from a unit area of a non-printing area is 0.18 mm.sup.3 /cm.sup.2). This lowers the press life of offset printing plates which for the above-mentioned offset plates does not exceed 100,000 prints.
Known in the art are processes for the manufacture of said printing plates comprising preparation of an aluminium plate. The preparation involves degreasing the surface of aluminium, pickling, graining by mechanical or electrochemical methods, oxidation of aluminium by means of direct current and application of an interlayer (or precoat) prior to application of a top layer. After preparation of the aluminium plate, a top layer is deposited thereon and an image is transfered thereto from a photo-mask-a diapositive in the case of a top layer based on derivatives of ortho-naphthoquinonediazide-sulphonic or carboxylic acids or from a negative in the case of a top layer comprising a photopolymerizable composition. After the image development, on the surface of aluminium non-printing areas are uncovered which are subjected to hydrophilization by treating with phosphoric acid. The printing areas comprise the top layer material (cf. French Application No. 2,187,938 Cl. C 23 b 9/02, B 41 n 1/08, French Pat. No. 2,187,622 Cl. B 41 n 1/04, B 41 n 3/04; FRG Pat. No. 2,622,799 Cl. G 03 F-H 02, FRG Pat. No. 1,914,054 Cl. 15 l 2/02).
These prior art process for the manufacture of offset printing plates have disadvantages residing in a multi-stage character of preparation of the aluminium plate surface. The necessity of such preparation is due, first of all, to the fact that the top layer is fixed on the aluminium plate surface only by adhesion forces, by reason of which the plate surface must have a certain microgeometry. Secondly, for a higher hardness of the aluminium plate surface, oxidation of aluminium is necessary for improvement of press life of non-printing areas. This operation is rather complicated and requires power-consuming galvanic operation necessitating powerful current sources and cooling equipment for lowering the temperature of the electrolyte during oxidation.
Known in the art are offset printing plates comprising an aluminium or steel substrate having printing and non-printing areas thereon which areas are made of two different metals. Usually the printing areas are made of copper and the non-printing ones of chromium. The non-printing areas are positioned above the printing areas at a height of 1-2 .mu.m. Between the substrate and copper layer there is an intermediate nickel interlayer or a non-metal contact film (cf. French Pat. No. 2,160,089 Cl. B 41 n 1/00, French Pat. No. 2,192,498 Cl. B 41 n 1/00; E. A. Nikanchikova, A. L. Popova "Offset Printing Process", "Kniga" Publishing House, Moscow, 1978).
Non-printing areas made on a harder and wear-resistant metal chromium feature a higher press life (up to 1 mln prints) and abrasion-resistance (the volume of the ground-off metal from a unit area of the non-printing areas is 0.045 mm.sup.3 /cm.sup.2). These offset printing plates have the disadvantage residing in the complicated structure thereof: plates for such forms consist of three basic metal layers (substrate, layer-carrier for non-printing areas and a layer-carrier for printing members) and interlayers ensuring the required adhesion of the basic metal layers to the substrate.
Known in the art are processes for the manufacture of the above-mentioned offset printing plates involving preparation of the surface of a steel or aluminium substrate and build-up of copper and chromium layers thereon. The substrate preparation comprises degreasing by chemical (for aluminium) or electrochemical (for steel) methods, followed by application, thereon of an interlayer to ensure the required adhesion of the subsequent layers. Then onto the prepared substrate there are deposited, electrolytically, first a copper and then a chromium layer to a predetermined thickness using electrolytes having a multi-component composition. Onto the thus-produced multi-metal plate there is deposited a light-sensitive top layer comprising PVA or a photopolymerizable composition. Then the top layer is dried and an image is transferred thereon from a diapositive. The image is developed, whereby the untanned regions of the top layer are removed. The uncovered regions of the multi-metal plate are etched (chemically or electrochemically), to the underlying copper layer. Then the tanned regions are removed from the top layer chemically or electrochemically to reveal chrome regions. Then copper printing areas are subjected to hydrophobization and chromium non-printing areas are subjected to hydrophilization using appropriate solutions (cf. French Pat. No. 2,160,089 Cl. B 41 n 1/00, E. A. Nikanchikova, A. L. Popova "Offset printing Process", "Kniga" Publishing House, Moscow, 1978).
The above-discussed prior art processes for the manufacture of offset printing plates have a disadvantage residing in a multi-stage character for the production of multi-metal plates and the necessity of using solutions of toxic agents for this purpose, as well as a need for special equipment comprising galvanic baths, current sources, ventilation devices, units for neutralization of waste waters. The process for the manufacture of offset printing plates on multi-metal plates is also complicated. Furthermore, etching of chromium to a copper layer on the image-bearing regions inevitably causes distortion of the tone rendering, since side etching occurs along with chromium etching into the depth. The top layer should meet severe requirements: it should be sufficiently acid-resistant in order to protect non-printing areas during acid etching.